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Technology: Gut power fuels cheaper coal oil

By ANDY COGHLAN

Bacteria that live in the human gut have provided the enzymes for a
cheap and simple system that breaks down coal into oil. Charles Scott and
his colleagues at the Oak Ridge National Laboratory in Tennessee say that
their system can liquefy as much as two-fifths of the coal fed into it.
Since the solid residue can still be used as a fuel, nothing is wasted.

Systems that liquefy coal do exist already, but they need temperatures
up to 500 degreeC and pressures of several thousand atmospheres. Scott’s
liquefier operates at less than 40 degreeC and at atmospheric pressure,
and so consumes much less energy.

His system works under such mild conditions because enzymes catalyse
the reactions. He acquired the enzymes, called hydrogenases, from a variety
of sources. One came from Proteus vulgaris, a common bacterium that lives
in the digestive tracts of mammals, including people. Another hydrogenase,
supplied by the University of Georgia, came from a deep-sea trench near
a hot ocean spring, and works at temperatures of up to 100 gegreeC. A third
is cytochrome C, which is vital to plants.

Scott first grinds up the coal into granules about 100 micrometres across,
then places them in a tapered column about 15 centimetres high. He dissolves
the enzymes in an organic solvent such as benzene or pyridine and squirts
the solution upwards, through the coal particles, along with hydrogen gas.

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As the fluid surges upwards in the column, the particles of coal swirl
and bubble, bringing them into contact with the enzymes. The enzymes catalyse
a reduction reaction in which the coal reacts with the hydrogen gas to form
a liquid. This oil dissolves in the organic solvent and can be siphoned
off for extraction.

Scott used an organic solvent because neither the coal particles nor
the liquid products dissolve readily in water. But this caused problems
because enzymes do not dissolve in oily organic solvents. This was solved
by modifying the enzymes chemically. Scott reacted them with dinitro-fluorobenzene
which added chemical groups to the enzyme molecules so they could dissolve
in the organic solvents.

So far, Scott has not worked with vessels larger than half-a-litre,
and he cautions that it will take at least a decade to commercialise the
system. He says that grinding the coal and pumping the solvent are the
most energy-hungry aspects of the process, but stresses that since no heat
has to be added the system is very efficient. Of the coals tried so far,
bituminous has worked best. Scott has also tried lignite, or brown coal,
but no hard coals such as anthracites.